1. Field of the Invention
This invention is broadly concerned with a hybrid vehicle which has, as a power source, the combination of an internal-combustion engine, such as a gasoline or diesel engine, and an electric motor driven by electrical energy stored in a battery. More particularly, this invention relates to a power transmission system for such a hybrid vehicle including a hydraulic power transmission, such as a torque convertor, for controllably transmitting the engine power to the driving wheels.
2. Description of the Related Art
Vehicles conventionally use an internal-combustion engine, such as a gasoline or diesel engine, as a power source. The internal-combustion engine is known for its capability for continuously providing high power for long distance travel, but is also known for a noise problem and generation of exhaust gases containing pollutants such as NOx and CO.sub.2.
In the light of these disadvantages, a vehicle using an electric motor has attracted a great deal of public attention for its potential to prevent noise and air pollution. However, an electric vehicle must carry a heavy battery with a limited electric capacity and does not develop sufficient power for acceleration, high load or high-speed travel. The electric vehicle has the disadvantage that it must be charged several times for long distance travel.
In view of the above-mentioned disadvantages, some so-called hybrid vehicles which use both an internal-combustion engine and an electric motor have been proposed. The known hybrid vehicles may be divided into two types, one being a series type wherein the internal-combustion engine constantly runs to drive a generator which, in turn, generates electricity for the electric motor which powers the vehicle and the other being a parallel type wherein the electric motor and the internal-combustion engine both selectively connect to the driving wheels to power the vehicle.
Japanese Patent Application Laid-open No. 59-204402 discloses a typical hybrid vehicle wherein the outputs of both the internal-combustion engine and the electric motor are transmitted to the driving wheels through an automatic transmission provided with a torque convertor. A one-way clutch is provided intermediate the internal-combustion engine and the electric motor to limit the transmission of power from the internal-combustion engine to the electric motor. In this prior art system, the driving wheels may be driven only by the internal-combustion engine or electric motor or by the combination thereof and the battery may be charged either by means of the internal-combustion engine or by a regenerative braking system.
While a torque convertor facilitates starting and accelerating of the vehicle, it also represents a great power loss for the vehicle. Such a power loss may not be a severe disadvantage when travelling under power of the internal-combustion engine, but this disadvantage becomes more conspicuous when travelling under power of the electric motor, due to the limited energy storage capacity of the battery.
If the internal-combustion engine is provided with a torque converter for transmitting its power to the driving wheels and if the electric motor is adapted to be directly connected to the same driving wheels, the above-mentioned power loss problem may be solved, but the electric motor must then be located intermediate the torque converter and the differential gear unit, necessitating a new concept of design. When an electric motor is the power source, the vehicle must accommodate a bigger electric motor to develop a larger torque without a transmission gear device, thus requiring a larger space to carry the motor and a change of design for the vehicle. Even if a flat-type electric motor is employed to minimize this disadvantage, the mounting must inevitably be altered, which is also costly.